1. Field of the Invention
The present invention relates to a semiconductor laser device used for optical disk apparatus such as a CD-R/RW drive, a DVD-RAM drive, a MD drive and the like.
2. Description of Related Art
It has been tried to increase the recording speed of optical recording apparatuses, and, for example, CD-drives of 16-time speed recording have been put into practical use. In such an optical recording apparatus of a high recording speed, it is necessary to momentarily start up high output laser light emitting. As examples of laser devices capable of fulfilling such required characters, there are ridge type semiconductor laser devices.
FIG. 3 is a schematic sectional view of a conventional ridge type semiconductor laser device, showing a section perpendicular to the longitudinal direction of the device.
On a substrate 31, a lower cladding layer 32, an active layer 33 and an upper first cladding layer 34 are stacked in this order. On the upper first cladding layer 34, a ridge-shaped upper second cladding layer 37 is formed so as to extend in the longitudinal direction of the device. On both sides of the upper second cladding layer 37, current blocking layers 36 are formed. On the upper second cladding layer 37 and the current blocking layers 36, a contact layer 38 is formed in ohmic contact with the upper second cladding layer 37.
On the lower surface of the substrate 31 and the upper surface of the contact layer 38, n-side electrodes and p-side electrodes (not shown) are formed respectively.
At the time of laser light emission, electrons recombine with holes in the active layer 33 to emit light. When the substrate 31 and the contact layer 38 are formed of GaAs and the lower cladding layer 32, the active layer 33, the upper first cladding layer 34 and the upper second cladding layer 37 are formed of AlGaAs based materials, the laser light can pass portions formed of AlGaAs based materials. Within these portions, the laser light is distributed in the form of a beam. The sectional form of a laser beam LB3 at an end face of the device is shown in FIG. 3.
FIG. 4 is an enlarged schematic sectional view showing the structure in the vicinity of the active layer 33 of the ridge type semiconductor laser device of FIG. 3.
Between the lower cladding layer 32 and the active layer 33, a lower beam enlargement layer 41 is provided. Between the active layer 33 and the upper first cladding layer 34, an upper beam enlargement layer 42 is provided. By setting the refractive indexes of the lower beam enlargement layer 41 and the upper beam enlargement layer 42 to suitable values respectively, a laser beam LB3 is enlarged by suitable widths in upward and downward directions from the active layer 33 respectively. Usually, the upper and the lower beam enlargement layers 41 and 42 are so designed that the laser beam LB3 can be enlarged by substantially equal widths both in the upward and downward directions from the active layer 33 respectively.
The distance L3 between the lower surface of the lower cladding layer 32 and the lower surface of the active layer 33 is so set that the lower end of the laser beam LB3 is positioned in the vicinity of the lower surface of the lower cladding layer 32. Similarly, the distance L4 between the upper surface of the active layer 33 and the upper surface of the upper second cladding layer 37 is so set that the upper end of the laser beam LB3 is positioned in the vicinity of the upper surface of the upper second cladding layer 37. The distances L3 and L4 are substantially equal to each other. When the substrate 31 and the contact layer 38 are formed of materials capable of absorbing the laser beam LB3, the length from the lower end to the upper end of the laser beam LB3 (hereinafter referred to as xe2x80x9clongitudinal beam diameterxe2x80x9d) is set to be shorter than the distance between the lower surface of the lower cladding layer 32 and the upper surface of the upper second cladding layer 37. Thus, the laser beam LB3 can be prevented from being absorbed.
The current injection from the upper second cladding layer 37 into the active layer 33 occurs through the lower surface of the upper second cladding layer 37. Therefore, in the active layer 33, the recombination of electrons and carriers of holes causes light emission in a region E3 (hereinafter referred to as xe2x80x9clight emission region E3xe2x80x9d) having a width substantially equal to the width S3 of the lower surface of the upper second cladding layer 37 (hereinafter referred to as xe2x80x9cinjection current width S3xe2x80x9d). In this case, carriers are not consumed uniformly throughout the light emission region E3 but consumed in a larger amount in a portion in the vicinity of the center, in the direction of the width, of the light emission region E3.
Though the amount of carrier consumption is nonuniform as mentioned above, the carrier density in the light emission region E3 can be kept uniform to some extent by diffusion of carriers (especially minority carriers). However, when the operation current is enhanced in order to obtain a high optical output, carriers cannot be sufficiently complemented by diffusion in case of the injection current width S3 being large. As a result, the carrier density becomes nonuniform in the direction of the width of the light emission region E3, and consequently the brightness distribution becomes nonuniform. That is, in the light emission region E3, the portion of light emission at a high brightness moves to a portion having a high carrier density, and consequently, the optical output becomes unstable. Since laser devices having unstable optical output cannot be used for optical recording and optical reading, the maximum rated optical output is determined within a range in which the optical output is stable.
By narrowing the injection current width S3, the problem of the nonuniform carrier density can be solved. That is, by making the injection current width S3 smaller than the distance of the carrier diffusion in the active layer 33 (carrier diffusion length), the portion in which a large amount of carriers are consumed is surely supplemented with minority carriers, so that uniform light emission in the light emission region E3 can be obtained. With such a structure, the optical output can be stabilized, whereby the maximum rated optical output of the laser device can be increased.
However, as the injection current width S3 is made narrower, the upper surface width D3 of the upper second cladding layer 37 also becomes narrow. The reason is that, since the side surfaces of the upper second cladding layer 37 are formed with a given inclination angle, the injection current width S3 and the upper surface width D3 cannot be separately changed in case of the thickness of the upper second cladding layer 37 being fixed. Since the current flows through the boundary surface between the contact layer 38 and the upper second cladding layer 37, the area of the boundary surface between the contact layer 38 and the upper second cladding layer 37 becomes small when the upper surface width D3 becomes narrow, so that the resistance value of the laser device rises.
An object of the present invention is to provide a semiconductor laser device capable of raising the maximum rated optical output without increasing the device resistance.
A semiconductor laser device according to the present invention includes a lower cladding layer, an active layer and an upper first cladding layer stacked in this order on a compound semiconductor substrate, a ridge-shaped upper second cladding layer provided on the upper first cladding layer, current blocking layers provided at the sides of the upper second cladding layer, and a contact layer provided on the upper second cladding layer. The distance between the upper surface of the active layer and the upper surface of the upper second cladding layer is shorter than the distance between the lower surface of the lower cladding layer and the lower surface of the active layer.
Laser light can pass through each of the layers from the lower cladding layer to the upper second cladding layer. The distance between the lower surface of the lower cladding layer and the upper surface of the upper second cladding layer is hereinafter referred to as xe2x80x9cpassable layer thicknessxe2x80x9d.
According to the present invention, the distance between the upper surface of the active layer and the upper surface of the upper second cladding layer is shorter than the distance between the lower surface of the lower cladding layer and the lower surface of the active layer. Therefore, the thickness of the upper second cladding layer according to the present invention can be designed thinner than that of a conventional device without changing the passable layer thickness. As a result, even if the injection current width according to the present invention is made narrower than that of the conventional device, the upper surface width of the upper second cladding layer can be of a size similar to that of the conventional device.
Therefore, by narrowing the injection current width, the nonuniform carrier density in the light emission region can be reduced without increasing the device resistance. Thereby, it is possible to enlarge the operation range in which the optical output is stable, so that the maximum rated optical output can be raised.
In order to making the passable layer thickness to be of a size similar to that of the conventional device, the thickness of the lower cladding layer has only to be made thicker than that of the conventional device. Thereby, the longitudinal beam diameter of the laser beam distributed in the passable layer thickness can be of a size similar to that of the conventional device.
In addition to the upper first cladding layer, other layers may be provided between the active layer and the upper second cladding layer. For example, an etching stop layer used for forming the upper second cladding layer into a ridge shape may be provided between the upper first cladding layer and the upper second cladding layer.
It is preferable that the bottom surface width of the upper second cladding layer is smaller than the diffusion length of carriers in the active layer. By making the injection current width smaller than the diffusion length of carriers, the region in the active layer in which many carriers are consumed is surely supplemented with minority carriers, so that uniform light emission can be obtained in the light emission region. As a result, the optical output can be stabilized, therefore; the maximum rated optical output can be raised.
It is preferable that a lower beam enlargement layer, having a refractive index lower than each of the refractive indexes of the lower cladding layer and the active layer, is provided between the lower cladding layer and the active layer. Further, it is preferable that an upper beam enlargement layer, having a refractive index lower than each of the refractive indexes of the active layer and the upper first cladding layer, is provided between the active layer and the upper first cladding layer,
When the refractive index of the upper beam enlargement layer and the refractive index of the lower beam enlargement layer are lower than that of the upper first cladding layer and that of the lower cladding layer respectively, the laser beam is enlarged by suitable widths in the upward and downward directions respectively from the active layer.
When, in a structure of a conventional ridge type semiconductor laser device, an upper second cladding layer is made to have a smaller thickness, the upper end of a laser beam reaches, beyond the upper surface of an upper second cladding layer, a region in which a contact layer exists. For example, an active layer, an upper first cladding layer and the upper second layer are formed of AlGaAs based material and the contact layer is formed of GaAs, a laser beam entering the contact layer is absorbed. Therefore, a laser beam generated in the active layer cannot be efficiently taken out of the device. Further, the laser beam taken out of the device cannot have a predetermined longitudinal beam diameter.
According to the present invention, a laser beam can be distributed narrower in the upward direction from the active layer than in the downward direction therefrom by means of the upper beam enlargement layer and the lower beam enlargement layer. Therefore, even if the upper second cladding layer has a small thickness, a laser beam enlarged in the upward direction from the active layer can be prevented from reaching the contact layer beyond the upper surface of the upper second cladding layer. That is, since the laser light is not absorbed by the contact layer, the laser beam can be efficiently taken out of the device.
Further, it is possible to make the distance between the lower surface of the lower cladding layer and the lower surface of the active layer longer than that of a conventional device and thereby to widely distribute the laser beam in the downward direction from the active layer, so that the passable layer thickness and the longitudinal beam diameter of the sizes respectively similar to those of the conventional device can be achieved.
It is preferable that the difference between the refractive indexes of the lower cladding layer and the lower beam enlargement layer is larger than the difference between the refractive indexes of the upper first cladding layer and the upper beam enlargement layer.
The smaller the difference between the refractive indexes of the upper beam enlargement layer and the upper first cladding layer becomes, the smaller the upwardly enlarged width of the laser beam becomes. The smaller the difference between the refractive indexes of the lower beam enlargement layer and the lower cladding layer becomes, the smaller the downwardly enlarged width of the laser beam becomes. Therefore, by making the difference between the refractive indexes of the lower beam enlargement layer and the lower cladding layer larger than the difference between the refractive indexes of the upper beam enlargement layer and the upper first cladding layer, the laser beam can be distributed more widely in the downward direction than in the upward direction from the active layer.
It is preferable that the thickness of the lower beam enlargement layer is larger than that of the upper beam enlargement layer. The larger the thicknesses of the lower beam enlargement layer and the upper beam enlargement layer are, the larger the enlarged width of the laser beam becomes. Therefore, by making the thickness of the lower beam enlargement layer larger than that of the upper beam enlargement layer, the laser beam can be distributed more widely in the downward direction than in the upward direction from the active layer.
The compound semiconductor substrate may be a GaAs compound semiconductor, and the lower cladding layer may be an Alx1Ga(1-x1) As layer.
The active layer may be a single layer of Aly1Ga(1-y1) As or a composite layer of an Aly11Ga(1-y11) As layer and an Aly12Ga(1-y12) As layer (for example, obtained by alternately stacking these two kinds of layers), or a composite layer of an Aly1Ga(1-y12) As layer and a GaAs layer (for example, obtained by alternately stacking these two kinds of layers). When the active layer is a MQS (Multi Quantum Well) active layer, it may be such a composite layer as mentioned above.
The upper first cladding layer may be an Alx2Ga(1-x2) As layer. The ridge-shaped upper second cladding layer may be an Alx3Ga(1-x3) As layer, and the current blocking layer may be an Aly2Ga(1-y2) As layer or a GaAs layer. The contact layer may be a GaAs layer.
The lower beam enlargement layer may be an Alz1Ga(1-z1) As layer and the upper beam enlargement layer may be an Alz2Ga(1-z2) As layer.
In a process of manufacturing such a semiconductor laser device, after forming the lower cladding layer, only by continuously stacking a layer of the AlGaAs based material with altering the Al content of the layer, the lower beam enlargement layer can be formed. Similarly, after forming the upper first cladding layer, only by continuously stacking a layer of the AlGaAs based material with altering the Al content of the layer, the upper beam enlargement layer can be formed. In an AlGaAs based material, the higher the Al content becomes, the lower the refractive index becomes. Therefore, by controlling the Al content, the refractive index of each part formed of an AlGaAs based material can be set to a desired value.
The above-mentioned and other objects, features and effects of the present invention will become more apparent from the following description of the embodiments given with reference to the appended drawings.